PHILADELPHIA  A document signed by President Abraham Lincoln that marked the official start of the Civil War is for sale.

The Raab Collection in Philadelphia said Tuesday it is selling the document, which it calls one of the most important in American history. The asking price is $900,000.

Lincoln’s proclamation ordering Union blockades of Southern ports is dated April 19, 1861. That’s a week after the first shots of the conflict were fired at South Carolina’s Fort Sumter.

After the Civil War ended in 1865, the U.S. Supreme Court called Lincoln’s blockade proclamation the official start of the war.

The document has been owned by a private collector. It has been exhibited at the National Constitution Center in Philadelphia and the Abraham Lincoln Presidential Museum and Library in Springfield, Ill.